Spinal fusion is one of the most commonly performed surgical procedures within the US and in Europe. The goal of spinal fusion surgery is to introduce bone growth between two or more vertebrae, fusing them into a single, continuous unit. Spinal fusion surgery is performed in the lumbar, cervical and thoracic regions, and fusions within each region are associated with a different set of complications. Even so, most complications following spinal fusion can be generalized into two broad categories: non-fusions, where the vertebrae are not fused into a singular unit due to insufficient bone formation within the fusion space; heterotopic ossification, where bone growth damages or impinges on tissue causing harm or discomfort to the patient. Examples of heterotopic ossification includes: Anterior osteophyte formation causing mass effect on the esophagus leading difficulty of swallowing (cervical fusions); ossification of the posterior longitudinal ligament; and formation of posterior osteophyte and/or other excessive posterior bone growth pressuring the spinal cord and/or spinal nerves.
Many contemporary spinal fusion hardware and biologics include designs to address the problems associated with non-unions, with little regard to heterotopic ossification. For example, commonly used biologics, particularly recombinant human bone morphogenetic protein (rhBMP-2), have been used to reduce non-fusion rates by increasing bone formation in the fusion space and the volume surrounding it. While clinically proven to decrease non-unions, numerous studies have shown that the biologic causes a host of side effects including but not limited to cancer, tissue swelling, growth of benign tissue, teratogenicity, pathological heterotopic ossification, nerve injury and spinal cord injury. While not all side effects caused by rhBMP-2 are related to heterotopic ossification, many are. As such, the biologic represents an illustrating example of how, nonspecific, unguided osteoinduction can be harmful to a patient and the delicate balance between increasing fusion rates and avoiding heterotopic ossification.
A second method utilized in reducing non-union rates is electrical stimulation. When mechanical stress is exerted on bone, an electric field is created. In the body, this electrical field constitutes a signal causing a physiological response resulting in osteoinduction or osteolysis. Consequently, it is possible to cause osteoinduction or osteolysis by introducing an electrical field in the volume within and surrounding a segment of bone. In volumes where the current density is above a certain threshold, osteoinduction is achieved if the polarity of the field in the region is electronegative while bone in regions where the field is electropositive undergoes osteolysis.
Currently, some risk of non-unions may be reduced using implantable or external electrical stimulators. Many existing implantable stimulators use hermetically-sealed, constant-current DC power sources attached to one or more electrodes, which can be large, unwieldy and prone to infection or complications. In addition, the electrodes of these implantable systems are long and are liable to break. Since implantable systems are designed to be placed along the length of the spine covering multiple vertebrae, they often migrate and may cause injury.
External stimulators are marketed by Biomet, Orthofix International and DJO Global. External stimulation systems use an AC signal generator connected to electrodes placed on the skin or to an induction coil, which introduces electrical fields in the volume of the spine and the volume enveloping it through induction.
The above mentioned systems suffer product specific disadvantages such as the complicated implantation procedures for the implantable systems and stringent patient compliance requirements for the external stimulators. Additionally, the risk of heterotopic ossification is not addressed by any present system. Specifically, similarly to rhBMP-2, existing electrical stimulation systems are aimed at reducing non-union rates through nonspecific, unguided osteoinduction.
Thus, there is a need in the orthopedic medical device field to create a new and useful system and method for dynamically stimulating bone growth. This invention provides such a new and useful system and method.